The present invention relates to a method of ejecting microdroplets of ink, and a particularly to such a method employed in an inkjet head driving method for applying pressure to ink in ink pressure chambers to eject microdroplets of ink from nozzles in communication with the ink pressure chambers.
A drop-on-demand inkjet technology well known in the art ejects ink droplets by applying a drive voltage waveform to piezoelectric elements. Inkjet printers employing this method render diverse colors on a recording medium by forming clusters of dots in a limited number of ink colors on the recording medium. Consequently, images formed by these types of inkjet printers tend to be particularly grainy in the highlights. Studies have been conducted on reducing the size of the ejected ink droplets in order to reduce the size of the dots formed on the recording medium and obtain higher image quality with no graininess.
Further, there have been studies conducted in recent years on using inkjet technology to form integrated circuits through patterning with conductive ink and to form a variety of thin films. Producing smaller ink droplets is also expected to be useful for forming high-density interconnects and uniform ultrathin films.
Certainly the size of ejected ink droplets can be easily reduced by reducing the diameter of the nozzles. However, high accuracy of the nozzles resulting from reducing the nozzle diameter leads to higher production costs. Further, the smaller nozzle openings become clogged more easily with foreign matter and ink deposits, leading to ejection problems.
However, one method enables the ejection of ink droplets that are smaller than the nozzle diameter by controlling oscillations of the ink surface in the nozzle opening (hereinafter referred to as the “meniscus”).
Japanese Patent Application Publication No. HEI-4-36071 discloses a method of ejecting small ink droplets by rapidly drawing in and holding the meniscus, causing the ink to rebound in the center of the meniscus and form a small ink droplet that is ejected therefrom. Japanese Patent No. 3,491,187 discloses a method of ejecting small ink droplets by drawing the meniscus far into the nozzle and subsequently contracting the chamber to generate and eject a narrow column of ink from only the center of the meniscus. Japanese Patent Application Publication No. 2000-141642 and Japanese Patent No. 3,159,188 disclose a method of reducing the size of ejected ink droplets by first drawing in the meniscus and then contracting the pressure chamber to form an ink column on the outside of the nozzle, and subsequently drawing in the meniscus again to reduce the volume of ejected ink.